This invention relates to a treatment process for bonding two structures by molecular adhesion, and unbending.
A structure is any micromechanical or integrated optical part, or microelectronic part that could be combined with another part by bonding. For example, this type of structure could be a substrate or a support board, equipped or not equipped with electronic, optical or mechanical components.
Furthermore, bonding by molecular adhesion refers to bonding that involves an interaction between chemical terminations present on the surfaces of structures in contact with each other.
The invention has applications particularly in the manufacture of devices with integrated circuits. In some manufacturing processes, semi-conductor boards containing integrated circuits must be combined with stiffening substrates, and then separated at the end of the treatment.
As mentioned above, and particularly in microelectronic applications concerning the manufacture of power circuits, semi-conductor wafers comprising integrated electronic circuits are in the form of large thin boards. For example, wafers with a diameter of four inches (xe2x89xa110 cm) and a thickness of less than 200 xcexcm are used.
Standard equipment for the manufacture of microelectronic devices, for example such as photorepeaters, are not suitable for the treatment of boards this thin. Furthermore, thin semiconductor boards are fragile, and this is incompatible with handling steps, and particularly handling using automated treatment equipment.
A thin board or a surface layer of a substrate with or without integrated circuits may be bonded on a treatment support also called a xe2x80x9chandling substratexe2x80x9d. The handling substrate thus provides it with sufficient mechanical strength for all required treatments and manipulations.
The attached FIGS. 1 to 3 described below illustrate transfer of a thin layer comprising integrated circuits, as an example.
The thin layer, marked in FIG. 1 as reference 10, is initially fixed to a substrate 12, called the source substrate. It comprises integrated electronic components and circuits, which are not shown.
The source substrate 12 and the thin surface layer 10 are transferred to a handling substrate 14 by bonding the thin surface layer on the handling substrate. The structure thus obtained is shown in FIG. 1.
The source substrate is then eliminated by a process such as grinding or cleavage, by etching and/or polishing to obtain the structure shown in FIG. 2.
The thin layer 10 comprising integrated circuits is then bonded upside down on the handling substrate 14. The handling substrate thus provides this layer with the stiffness necessary for other manufacturing operations or treatments.
In a final step shown in FIG. 3, the thin layer 10 containing the electronic circuits is transferred to a target substrate or a destination substrate 16, onto which it is permanently fixed.
After attachment to the destination substrate 16, the thin layer 10 is separated from the handling substrate 14. Thus the handling substrate 14 is shown in dashed lines in FIG. 3.
This type of process is described in more detail in document (1), for which the reference is given at the end of this description.
The thin layer 10 may be bonded on the handling substrate 14, for example cold using an appropriate glue. Bonding is then reversible and it is possible to separate the thin layer 10 from the handling substrate. However, the adhesion obtained between the thin layer 10 and the handling substrate 14 may be insufficient, particularly for subsequent treatments at high temperature. In particular, the glue is incapable of resisting high temperatures.
Furthermore, the material (glue) added for bonding can cause metallic or organic contamination of bonded parts during subsequent treatments.
These disadvantages are avoided by preferring bonding by molecular adhesion which does not use any glue or added material. Bonding two structures by molecular adhesion includes four main steps, which are described below.
A first step is surface preparation of the structures to be brought into contact. A good quality molecular bonding requires control of important parameters such as surface roughness, which should preferably be less than 0.5 nm (4 xc3x85) as a root mean square value, the lack of any dust (particles  greater than 0.2 xcexcm) on surfaces, the planeness of the surfaces to be put in contact, and the chemical state of these surfaces.
Thus the first step consists mainly of cleaning the surfaces of structures to be bonded in order to eliminate foreign particles and to make these surfaces hydrophile.
FIG. 4 shows a structure for bonding comprising a silicon substrate 20, one surface 22 of which has been made hydrophile. Surface 22 comprises a first hydrophile layer 24 composed essentially of Sixe2x80x94OH chemical groups and one (or several) layers of water H2O 26 adsorbed on the hydrophile layer 24.
A second step consists of putting the hydrophile surfaces of the two structures to be bonded into contact. Putting them into contact brings the water layers adsorbed on these structures sufficiently close together for them to interact with each other. The attraction exerted between the water molecules is propagated gradually along the entire surface of each structure. The surfaces in contact are then bonded together.
The bonding energy as measured by a blade insertion method is of the order of 0.15 J/m2. This value is typically the value of hydrogen type adhesion between two water layers, on each structure.
Document (2), the reference of which is given at the end of this description, contains an illustration of the blade insertion method.
A third step consists of solidification heat treatment of the adhesion.
The heat treatment can eliminate water layers between the assembled structures, up to a temperature of the order of 200xc2x0 C.
Adhesion of structures then takes place by bonding of OH groups between the layers of Sixe2x80x94OH chemical groups in each structure, respectively. Note that the layer of Sixe2x80x94OH groups is shown as reference 24 in FIG. 4. This interaction results in a reduction of the distance between the two structures in contact and results in the interaction of additional OH groups. The bonding energy thus increases for treatment temperatures of 200xc2x0 C. to 900xc2x0 C.
Finally, there may be a fourth step consisting of heat treatment at more than 900xc2x0 C. In this step, the interacting Sixe2x80x94OH groups change towards Sixe2x80x94Oxe2x80x94Si type bonds, which are much stronger. This then gives a very strong increase in the bonding energy.
The graph in FIG. 5 shows the bonding energy per unit area between structures bonded by molecular adhesion as the ordinate, as a function of the treatment temperature. Bonding energies are expressed in J/m2 and temperatures are expressed in xc2x0C.
Regions 32, 33 and 34 in the graph are related to the second, third and fourth steps in the bonding process and correspond to a hydrogen type interaction between water films, a hydrogen interaction between OH groups (reference 24), and then an Sixe2x80x94Oxe2x80x94Si type interaction, respectively. A more detailed description of bonding of silicon wafers may be found in document (3), the reference of which is given at the end of this description.
Note that at treatment temperatures above 600xc2x0 C., it becomes impossible to unbond the two assembled structures without causing severe degradation to them.
When the assembled structures are silicon boards, bonding energies greater than 2 J/m2 may be obtained. These energies are thus of the same order of magnitude as the cohesion energies of the silicon material.
It is immediately clear that if molecular bonding is used in a transfer process like that shown in FIGS. 1 to 3, it will be impossible to detach the handling substrate from the thin layer by applying mechanical forces, without destroying the thin layer or the handling substrate.
Thus, the thin layer is separated from the handling substrate by eliminating the handling substrate. For example the handling substrate can be eliminated by grinding and/or mechanical-chemical abrasion.
In this case, the process for transferring a thin layer involves the sacrifice of a handling layer for each treated thin layer. This sacrifice also introduces a large industrial cost.
The purpose of this invention is to propose a treatment for bonding of two structures which can firstly give a very strong molecular bond between the two structures, and will also enable unbonding of the structures along the bonding interface.
Another purpose of the invention is to propose a treatment enabling unbending that does not damage the assembled structures.
More precisely in order to achieve these objectives, the purpose of the invention is a treatment process for bonding two structures by molecular adhesion on a bonding interface, and for separation of the two structures along the said bonding interface.
In accordance with the invention,
bonding is done using at least one structure containing at least one element capable of diffusing within the said structure to the bonding interface, and
a heat treatment is used for unbonding, with a sufficient heat budget to make the said element diffuse towards the bonding interface to weaken it.
An element capable of causing diffusion refers to any element or compound either intrinsically present in the material or added to it, deliberately or accidentally, capable of migrating within the material towards the bonding interface, to react with it. This element is then capable of modifying this interface during the heat treatment and will cause separation of the two parts on each side of the interface. This separation may be assisted by a gaseous phase which may form at the interface during the heat treatment.
Furthermore, heat budget means the sum of heat treatments carried out and defined by a time/temperature pair applied to the structure.
Thus, the heat treatment designed to separate the two parts (on each side of the bonding interface) may take account of heat treatments applied to the assembled structures before unbending.
According to one particular embodiment of the process, a hydrogen implantation may be done before bonding in at least one of the structures, the hydrogen forming the said element capable of diffusing in the structure.
For example, implantation is done in silicon with a dose of between 1016 and 5.1016 (H+/cm2) and an energy of between 20 and 500 keV. Preferably, the dose may be of the order of 3.1016 ions/cm and the implantation energy of the order of 70 keV. The dose depends on the implantation conditions and particularly the temperature of the structure during the implantation.
According to one variant, at least one structure may also be used comprising a surface oxide layer formed by plasma enhanced chemical vapor deposition and containing OH molecules, the said OH molecules forming the element capable of diffusing.
For example, the heat treatment for unbonding may be done at a temperature of between 600 and 1350xc2x0 C. for silicon. This temperature would be chosen to be of the order of 200 to 600xc2x0 C. for gallium arsenide (AsGa). For silicon carbide (SiC), the chosen temperature will be between 600xc2x0 C. and the melting temperature which exceeds 1350xc2x0 C.
For example, the heat treatment may take place under heating lamps or in a furnace.
The structures to be assembled may be structures made of a single solid material, or may be multi-layer structures containing zones which may or may not have been treated.
The multi-layer nature of the structures may beneficially generate internal stresses that facilitate separation of structures during the unbonding step.
Similarly, the surface of at least one of the structures to be assembled may be prepared before bonding to form a relief. This relief may also facilitate separation of the structures when unbonding.
Finally, external separation forces may be exerted on the structures to further facilitate unbending. For example, tension or bending forces, or shear forces, may be exerted on the structures by inserting a blade at the interface between the structures.
Other characteristics and advantages of the invention will become clearer from the following description with reference to the figures in the attached drawings. This description is given for illustration only, and is in no way restrictive.